A 28-year-old laboratory worker at a plague research institute in Siberia has died after falling ill, prompting Russian authorities to quarantine about 200 contacts.

Darya Shipilova worked at the Irkutsk Anti-Plague Research Institute of Siberia and the Far East. She developed pneumonia of unknown origin and died last week. Unconfirmed reports said a test tube containing the plague pathogen broke before she fell ill.

Russia’s health watchdog said no plague was found among her contacts. It said extensive testing found no lab-linked microorganisms in her body. About 60 percent of those quarantined have since been released.

The World Health Organization said it still does not know what pathogen caused the death and is seeking more information from Moscow.

US President Donald Trump said American officials do not believe the case involved a bioweapon. He added that Russian officials have assured Washington the situation is under control.

Plague is a bacterial disease usually spread through fleas or contact with infected animals. It can be treated with antibiotics when caught early, health experts say. The institute studies particularly dangerous infections found in the region.